Document the ARDUINO_* constants

[alranel]

The official Arduino cores define a set of constants that can be used to write more portable sketches and code by identifying the architecture and board being used.

We have architecture-specific constants such as:

• ARDUINO_ARCH_AVR
• ARDUINO_ARCH_SAMD

As well as board-specific constants such as:

• ARDUINO_GIGA
• ARDUINO_UNOR4_WIFI

We should add a page to the language reference documenting all the available constants.

[alranel]

Bonus: add a link to a dedicated page on best practices for writing portable code, showing how to use such constants for common board-specific tasks such as pin numbers or Wire object to use etc.